This is the way to do it.I have found the easiest way to carry a Brompton up and down stairs is to do it unfolded with the saddle hooked on your shoulder.
Clipped together around the frame.how does that work? Carabiners on the frame or clipped together to make a sling? A stout bag with a strap might be easier.
I travel fairly frequently with my Brompton on Eurostar. I do fold it so it goes through the x-ray machine. Once they insisted that I put it in the Dimpa bag before Security. I complied because otherwise I risked missing my train. However I later looked up the conditions of carriage and nowhere does it say the bike has to be bagged before you get on the train. So next time I will ask them to justify their request.occasionally, I have to carry it folded, such as when taking the Eurostar, where security insist the bike be folded and bagged even before passport check.
Probably best not to, because by appearing awkward over what could be characterised as a 'security issue' you are giving them an excuse to ruin your day by denying you carriage.So next time I will ask them to justify their request.